Btrfs devel - Jul 2012 - No/bad auto-detection of fs type for small volumes (related to mixed metadata/data?)

When I create a btrfs volume of size strictly less than 256 MiB then if I do
    mount /dev/sdb1 /mnt/test
the kernel tries unsuccessfully to do the mount with many other file systems
before successfully trying with btrfs. For volumes of size larger than
or equal to
256 MiB it just mounts the volume without doing that. Why is this discrepancy?

Another possibly related symptom is that the volume does not appear in
/dev/disk/by-label and /dev/disk/by-uuid at all. This means that it is
impossible
to mount the volume by uuid or label.

To make sure that this isn''t a udev bug, I booted my system with
init=/bin/bash
in the kernel command line, and then I tried again to mount the
volume. This time
it would not mount it at all unless I explicitly specified the fs
type. On the other
hand, it could mount larger volumes without any issues.

All the experiments were done in an initially zeroed out disk. I am
using 3.4.6 kernel
with btrfs from 3.5 and the latest btrfs-progs from git.
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On Tue, Jul 24, 2012 at 08:39:36PM -0400, Marios Titas
wrote:
> When I create a btrfs volume of size strictly less than 256 MiB then if I
do
>     mount /dev/sdb1 /mnt/test
> the kernel tries unsuccessfully to do the mount with many other file
systems
> before successfully trying with btrfs. For volumes of size larger than
> or equal to
> 256 MiB it just mounts the volume without doing that. Why is this
discrepancy?
   Are you using the --mixed option when creating the filesystem? If
not, you should do with something that small.

   Hugo.
> Another possibly related symptom is that the volume does not appear in
> /dev/disk/by-label and /dev/disk/by-uuid at all. This means that it is
> impossible
> to mount the volume by uuid or label.
> 
> To make sure that this isn''t a udev bug, I booted my system with
init=/bin/bash
> in the kernel command line, and then I tried again to mount the
> volume. This time
> it would not mount it at all unless I explicitly specified the fs
> type. On the other
> hand, it could mount larger volumes without any issues.
> 
> All the experiments were done in an initially zeroed out disk. I am
> using 3.4.6 kernel
> with btrfs from 3.5 and the latest btrfs-progs from git.
-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ==  PGP
key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
         --- In theory, theory and practice are the same. In ---         
                      practice,  they''re different.

On Tue, Jul 24, 2012 at 6:39 PM, Marios Titas <redneb8888@gmail.com>
wrote:
> When I create a btrfs volume of size strictly less than 256 MiB then if I
do
>     mount /dev/sdb1 /mnt/test
> the kernel tries unsuccessfully to do the mount with many other file
systems
> before successfully trying with btrfs. For volumes of size larger than
> or equal to
> 256 MiB it just mounts the volume without doing that. Why is this
discrepancy?
If I understand correctly, the kernel does not implement any fs
detection; this is performed by the mount utility, which indeed may
try a bunch of different filesystems until it finds one that works.

From man mount:
              If no -t option is given, or if  the  auto  type  is  specified,
              mount  will try to guess the desired type.  Mount uses the blkid
              or volume_id library for guessing the filesystem type;  if  that
              does not turn up anything that looks familiar, mount will try to
              read the file /etc/filesystems, or,  if  that  does  not  exist,
              /proc/filesystems.   All  of  the  filesystem types listed there
              will be tried, except for those that are labeled "nodev"
(e.g.,
              devpts,  proc and nfs).  If /etc/filesystems ends in a line with
              a single * only, mount will read /proc/filesystems afterwards.
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On Wed, Jul 25, 2012 at 5:09 AM, cwillu <cwillu@cwillu.com>
wrote:
> On Tue, Jul 24, 2012 at 6:39 PM, Marios Titas <redneb8888@gmail.com>
wrote:
>> When I create a btrfs volume of size strictly less than 256 MiB then if
I do
>>     mount /dev/sdb1 /mnt/test
>> the kernel tries unsuccessfully to do the mount with many other file
systems
>> before successfully trying with btrfs. For volumes of size larger than
>> or equal to
>> 256 MiB it just mounts the volume without doing that. Why is this
discrepancy?
>
> If I understand correctly, the kernel does not implement any fs
> detection; this is performed by the mount utility, which indeed may
> try a bunch of different filesystems until it finds one that works.
>
> From man mount:
>               If no -t option is given, or if  the  auto  type  is 
specified,
>               mount  will try to guess the desired type.  Mount uses the
blkid
>               or volume_id library for guessing the filesystem type;  if 
that
>               does not turn up anything that looks familiar, mount will try
to
>               read the file /etc/filesystems, or,  if  that  does  not 
exist,
>               /proc/filesystems.   All  of  the  filesystem types listed
there
>               will be tried, except for those that are labeled
"nodev"  (e.g.,
>               devpts,  proc and nfs).  If /etc/filesystems ends in a line
with
>               a single * only, mount will read /proc/filesystems
afterwards.
Thanks, that was helpful. It was a blkid bug. It was fixed  [1] in
util-linux 2.21.

[1] https://git.kernel.org/?p=utils/util-linux/util-linux.git;a=commit;h=04f7020
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

On Tue, Jul 24, 2012 at 08:39:36PM -0400, Marios Titas
wrote:
> When I create a btrfs volume of size strictly less than 256 MiB then if I
do
>     mount /dev/sdb1 /mnt/test
> the kernel tries unsuccessfully to do the mount with many other file
systems
> before successfully trying with btrfs. For volumes of size larger than
> or equal to
> 256 MiB it just mounts the volume without doing that. Why is this
discrepancy?
[As Hugo wrote, use --mixed in that case]

The underlying reason why it fails is that 256M is a size of some
internal structure. If the size is smaller, filesystem will not set up
itself, then the mount fails. And mount interprets this as if the
filesystem was not there and tries next one.


david
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs"
in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html